Stirling engines refer to a specific class of external combustion heat engines that convert heat differentials into mechanical energy with relatively high conversion efficiencies. Such efficiencies for a class of optimized Stirling engines can surpass most known air-breathing internal combustion engines, and utilize a regenerator to store or release fluid heat during the engine cycle. Such Stirling engines can approach the efficiency of an ideal Carnot cycle.
Heat engines can be used in a variety of applications. For example, as prime movers, cooling systems, cryogenic coolers, heat pumps or pressure generators in a variety of design forms and operating sizes. Current applications range in size from large power generators to miniature engines for artificial hearts. Stirling engines thus far developed vary in output power from as little as a few watts to as much as 1MW (1MW=1341 hp). High temperature Stirling engines may operate at temperatures exceeding 1000 K, with mean working fluid pressures in the range of one atmosphere to as high as 20 MPa (1 MPa xe2x96xa1 10 atmospheres).
Conventional Stirling engines utilize the roughly steady-state expansion of a highly compressed fixed number of light molecular mass working fluid, such as helium, hydrogen or air, in contact with a heat source at a substantially fixed temperature for their power stroke; followed by forced convection heat transfer or gas cooling by contact with a heat sink to generate engine speeds ranging from low to high frequency, typically measured in revolutions per minute.
Since the essential ingredient needed to operate a Stirling engine is an appropriate external heat source such as, for example, solar, natural gas, fossil fuel, oil, coal, waste heat or geothermal energy; this makes the Stirling engine well suited for not only terrestrial applications but also for large scale space and underwater applications, including spacecraft and submarines.
Any conventional (non-rotary) type Stirling engine requires simple components for its operation. It requires internal pistons as the means for displacing and compressing the working fluid therein and to generate output power. The pistons receive work during their up-stroke (compression), and generate greater work during their down-stroke (expansion), followed by a transfer of heat at some temperature by the working fluid to the surrounding heat sink. The power pistons are usually equipped with high performance fluid rings to assure and maintain a high pressure differential between their upper and lower faces.
Regenerators, which are placed between the hot and cold heat exchangers, optimally recycle the heat supply and transfer process by acting as thermodynamic sponges. Their function is to receive heat from the working fluid during the fluid passage from the high to low temperature space, and to release heat to the working fluid during the fluid passage back from the low to high temperature space.
Generally, the system efficiency and the cyclic work output are functions of both thermodynamic variables, such as pressure, and the internal volumetric compression ratio. From a thermodynamic standpoint, an ideal reversible four-path Stirling cycle when depicted in the pressure (P) versus volume (V) diagram consists of two isothermal (constant temperature) and two isochoric (constant volume) processes in sequence. When depicted in the temperature (T) versus entropy (S) diagram, the heat energy transfer in the process is proportional to the area enclosed in the T-S diagram (SdT). Likewise, the work done by the engine is proportional to the area enclosed on the P-V diagram (PdV).
Disadvantages of conventional Stirling engines include the use of relatively expensive and heavy materials, such as Inconel(copyright) and other alloy steels, for the high temperature structural components (e.g., pistons, cylinders and regenerators). In addition, the use of seals at the piston connecting rod is a serious factor for limiting the useful life of the engine and is a well-known cause for downgrading the engine""s overall efficiency. Further, these seals are specialized designs and are correspondingly expensive to produce, and typically do not perform an adequate function in preventing leakage of high-pressure working fluid.
Conventional high temperature Stirling engines generally operate at high rotational velocities of typically about 3000 RPM. This is one prime reason for the reduction of the regenerator efficiency, and causes a marked increase in frictional losses from the high velocity motion of the working fluid. Other adverse effects due to high temperatures, typically about 720xc2x0 C., include the high heat losses due to the blackbody radiation. Although the low to middle temperature types of Stirling engines ( less than 450xc2x0 C.) can alleviate some of these losses, the price to pay is a lower Carnot efficiency. There also remain numerous other drawbacks, deficiencies, and disadvantages associated with conventional Stirling engines. One disadvantage is the premature failure of the seal between the connecting rod, which exhibits complex translational and rotational motions, and the mechanical drive linkage, despite expensive seal designs. A further disadvantage is the mechanical coupling of all adjacent pistons that results in a fixed phase angle relationship, which prevents optimization of the engine.
Since a Stirling engine is a device based on an oscillatory and forced convection of the working fluid, parasitic losses of the engine are related to the frequency of the operation. The higher the frequency the worse are certain performance losses. For a fixed target power, the lower frequency engine may be preferable. Reduction of unnecessary dead space volumes that do not participate in power generation and overall operation is desirable in the design of an optimized Stirling engine.
The above-described and other problems or disadvantages of the prior art are overcome or alleviated by a fluidic piston engine in which a fluidic piston is in fluid communication with a mass of compressible working fluid; and a second piston is in hydraulic communication with the fluidic piston and in fluid communication with the working fluid.
These and other features are further exemplified by an external combustion engine comprising at least four upright cavities disposed substantially equidistant from a central upright axis, a compressible working fluid in fluid communication between each pair of adjacent cavities; and at least one linkage in reciprocal phase communication between each pair of alternate cavities.